The diminished size of structures in modern semiconductor devices places extreme demands on semiconductor processing equipment. For example, the use of 0.35 .mu.m design rules requires very low levels of particulate contamination throughout the processing environment. Recently, in situ particle monitors have been identified as potentially useful tools for monitoring the performance of semiconductor processing equipment. It is believed that, under certain circumstances, the output of in situ particle monitors can be correlated with the presence of undesirable levels of particulates in the reaction environment. Both developmental and production line semiconductor processing equipment may incorporate laser-based optical systems for monitoring particulate levels in exhaust gas flow as a measure of equipment cleanliness.
During processing, particulates created in the equipment chamber are withdrawn from the chamber and flow through the exhaust line of the chamber. Typically, particle monitors are laser-based optical systems designed to detect the low levels of scattered light produced when the laser light interacts with the particulate matter in the exhaust line flow. Sustained operation of such particle monitoring sensors is made difficult by the harsh chemical environment that exists in the exhaust line flow of semiconductor processing equipment. Chemical species within the exhaust gases deposit on the walls of the exhaust lines, and these species may deposit on or etch surfaces that are not suitably inert to the exhaust gas environment. Optical surfaces which face the interior of the exhaust line, such as those necessary for a low light level optical detection system, are quickly degraded in such an environment.
Increased scattering in the particle monitoring sensor's optical path may significantly diminish the signal to noise ratio for the sensor and thereby render it ineffective. For example, particulate matter deposited on the detector windows can reduce the level of light gathered by the collection optics, reducing the total signal level. In addition, a poor optical surface on the laser inlet window can scatter the input laser beam and the scattered beam may enter the detector, introducing an optical noise signal into the particle monitoring sensor. This noise signal can be so strong that no particle detection would be possible. As a consequence, the reactor must be taken out of service frequently to service the particle monitoring sensor by cleaning or replacing the optical surfaces. Because existing particle monitoring sensors are often integral parts of the reactor exhaust system, such servicing renders the host equipment inoperable and reduces the practicality of particle monitoring sensors.
One solution to the problem of degraded optical surfaces in exhaust line particle monitoring sensors is discussed in U.S. Pat. No. 5,083,865, entitled "Particle Monitoring System and Method." That patent describes a particle monitoring system which incorporates a number of strategies to limit the degradation of the optical surfaces in the described particle detector. One technique described is to direct a flow of clean gas across the optical surfaces of the particle detector. This gas flow impedes the diffusion of exhaust gases to the optical surfaces within the exhaust line. However, for particularly harsh environments, substantial flow rates across the optical surfaces are necessary to prevent damage to the optical surfaces. Such substantial flow rates are, in general, undesirable because the gas flowing into the exhaust line adversely affects the vacuum and flow characteristics of the reaction chamber.
Another technique described in the '865 patent limits the degradation of optical surfaces by heating the optical surfaces to suppress deposition from the exhaust gases onto the surfaces of the particle monitoring system optical surfaces. In the described system, a large heater is attached to the exhaust line upstream from the particle detection system. Heat is input to the exhaust line to heat the surfaces of the windows of that particle monitoring system. In practice, less heat than is input actually reaches the windows, probably because of the relatively low thermal conductivity of the stainless steel exhaust line. For high heating levels, an undesirable amount of heat may propagate through other parts of the particle monitoring system. This heat increases the operating temperature of both the semiconductor photodetector and the semiconductor laser diode, diminishing the performance and reducing the lifetime of both elements.